Feeding physiology of the cold water appendicularian Oikopleura Vanhoeffeni (Tunicata)

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Abstract

Clearance and ingestion rates, behavior, gut passage time, pigment degradation and assimilation efficiency were examined for Oikopleura vanhoeffeni (Appendicularia, Tunicata) in a gradient of concentrations of laboratory grown diatoms. Data from particle removal experiments and behavioral observations showed similar trends, although the magnitude of the responses varied depending on the technique used. Clearance rate decreased significantly with food concentration, although saturation was not reached for a particle concentration range representative of conditions in the field. No lower feeding threshold was found. Clearance rate also decreased with the age of the external filtering device (i.e. house). For an individual animal, a wide range of clearance rates can be encountered from 0 to a physiological upper limit given by the Morris and Deibel (1993) model. Using body size, tail beat frequency and proportion of time spent feeding from in situ observations it was possible to estimate clearance rate in the field. The means of these estimated clearance rates were within a threefold range of the means of five alternative methods. The mean gut passage time of O. vanhoeffeni was 0.8 h and was independent of trunk length and particle concentration, although it varied significantly among individuals. Studies with ⁶⁸Ge incorporated into the silica frustules of diatoms as a conservative tracer, showed that chlorophyll a conversion (i.e. degradation into fluorescent and non-fluorescent breakdown products) was on average 79%. The chl a conversion was not related to the amount of food in the gut estimated by visual inspection, although it was inversely correlated with the amount of ⁶⁸Ge and chl a recovered in the guts of animals. Assimilation efficiency of bulk diatom carbon was 67%. Extraction in various solvents allowed fractionation of food and feces into four main biochemical pools. Proteins and low molecular weight compounds were preferentially absorbed by the animals over lipids and polysaccharides. Predicted C:N ratios (by weight) for fecal pellets produced on a diatom diet ranged from 6.0 to 7.2, depending on the formula used and were consequently not much higher than the C:N ratios of the ingested diatoms (C:N = 5.2 - 6.3). The results of this thesis are relevant for the calculation of realistic population clearance rates as well as for the biochemical transformation of sinking material as appendicularians in general are major grazers in the world oceans.